Source:

Response:

The carcass was a partly decomposed basking shark, according to tissue
samples and descriptions and sketches by witnesses. Reports that it
was a plesiosaur were based on a superficial resemblance caused by the
pattern in which sharks decay and were spread and exaggerated by news
media hype (Kuban 1997).

Even if the carcass were a plesiosaur, the find would not have been a
challenge to evolution. The theory of evolution does not demand that
species go extinct after a certain amount of time.

Reference:

Kuban, Glen J., 1997. Sea monster or shark? An analysis of a supposed
plesiosaur carcass netted in 1977. Reports of the National Center for
Science Education 17(3): 16-28. http://paleo.cc/paluxy/plesios.htm